There is known a conventional signal switch as described in IEEE IEDM Tech. Digest 01, p921, 2001, for example. This is structured with a signal transmission line 2502 formed on a high-resistance silicon substrate 2501, a movable ground line 2503 arranged over the signal transmission line 2502 through a predetermined gap, and a ground line 2504, as shown in FIG. 1A. In this switch, a voltage is applied across a parallel plate capacitance comprising the movable ground line 2503 and signal transmission line 2502, whereby an electrostatic force is caused to put the movable ground line 2503 into contact with the signal transmission line 2502 through a high dielectric film 2505 as shown in FIG. 1B. By the contact, increased is the capacitance formed between the signal transmission line 2502 and movable ground line 2503, making it possible to transfer a signal having a frequency component dependent upon that capacitance.
By thus controlling the voltage between the movable ground line 2503 and the signal transmission line 2502, the signal transmission is connected and disconnected from the signal transmission line 2502 to the movable ground line 2503. Furthermore, with this scheme, a signal switch can be formed by the same process as an LSI fabrication process. By forming a signal switch at the same point as that of a circuit of transistors or the like, it is possible to form a switch advantageous in respect of frequency characteristic and size reduction.
As the means for improving the operation speed in both signal connection and disconnection, there is a proposal that a seesaw form is provided to drive the movable electrode in two directions, e.g. described in Jpn. J. Appl. Phys., Vol. 40, p2721, 2001. In IEEE MEMS 2002 Tech. Dig., p532,2002, there is also known a structure that a voltage is applied between a stationary comb electrode and a movable comb electrode, to rotate a reflection mirror.
The conventional switches require transmission efficiency in signal transmission, insulation capability upon disconnection and high-speed operation at signal connection and disconnection.
However, in the structure of FIG. 1, it is only the signal transmission line 2502 that acts to drive the movable ground line 2503. When the signal is switched from the transmission line 2502 to the ground line 2503, voltage is applied between the ground line 2503 and the transmission line 2502. However, in the case to disconnect a signal being conveyed to the ground line 2503, there is difficulty in increasing the switching speed, because the operation is carried out only by the spring returning force of a material structuring the ground line. In case the ground line 2503 uses a material having a high spring constant, it is possible to increase the switching speed in disconnecting the signal being conveyed to the ground line 2503. However, this involves problems, e.g. decreasing operation speed in switching from the transmission line 2502 to the ground line 2503, and requiring to increase the voltage to be applied to between the ground line 2503 and the transmission line 2502.
Meanwhile, in the process for fabricating the above structure, after forming the transmission line 2502, formed in a correct film thickness is a sacrificial layer that is formed by etching only a predetermined material without etching the transmission line 2502 and ground line 2503. Then, the ground line 2502 is formed. Thereafter, the sacrificial layer is removed between the transmission line 2502 and the ground line 2503, thereby accurately forming a predetermined gap. This is a general process in practice. According to this method, in case a three-layer structure is provided to further fix a movable contact line driving electrode on the ground line 2503, even when to disconnect the signal being conveyed to the ground line 2503, the ground line 2503 can be moved at a high speed.
However, such a three-layer structure requires to accurately form not only the below of the ground line 2503 but also a sacrificial layer above the ground line 2503, in the fabrication process. This makes the fabrication process complicated. Furthermore, in the case of the three-layer structure, a step is generated by comprising five layers, i.e. the transmission line 2502, sacrificial layer, ground line 2503, sacrificial layer and movable ground line driving electrode, in the fabrication process. It is practically impossible to carry out a process of forming a pattern or the like over such a high step.
Meanwhile, in the case of forming a switch by a beam structure as shown in FIG. 1B, stress is changed by a temperature change. This takes place where there is a difference in thermal expansion coefficient between the material structuring the beam and the material structuring a substrate. The beam stress change causes a change of beam spring constant, which in turn changes the switch response time and driving voltage. The beam, in the worst case, is known to be deformed 2 μm or greater by a temperature change. In order to achieve a high-speed response, the driving distance of the movable electrode must be set at a required minimum distance for obtaining a desired isolation. In this manner, the distance between the electrodes must be sufficiently long while taking into account the beam deformation amount by such a temperature change. This, however, further increases the response time.
On the other hand, in the case of a seesaw type, a capacitor capacitance is formed based on an overlap area of a signal electrode and a contact electrode. Because the magnitude of capacitance determines a transmission signal frequency and transmission efficiency, the size of the contact electrode is determined by a signal to be controlled in connection and disconnection. In order to obtain a connection/disconnection characteristic on a signal at a certain fixed frequency, it is impossible to reduce the size of the contact electrode. Furthermore, the entire mass of the movable electrode requires the part for forming a capacitor formed by a pull electrode and a push electrode, in addition to the contract electrode mass. As a result, there is needed to form an electrode at the part not directly involved in signal connection and disconnection, increasing the overall mass of the movable electrode. This is disadvantageous in connection and disconnection at a high speed.
In a driving scheme using a comb electrode, formation is comparatively easy for those for driving in an in-plane direction of a substrate. However, those for driving in a vertical direction to a substrate require to form a structure in a height direction, making the fabrication process complicated.